1. Field of the Invention
This invention relates to a circuit for grounding at multiple frequencies, more particularly, to a circuit for grounding at IF and at RF.
2. Description of the Related Art
Balanced mixers are employed to frequency shift a first signal at a first frequency to a second frequency. These mixers have a balanced set of inputs and a balanced set of outputs. In some applications employing a balanced mixer, only the first signal at the first frequency is available as an input signal. This input signal is provided to a first input of the balanced set of inputs for the mixer. In such applications, the second input of the set of balanced inputs is grounded to provide a balanced set of input signals. Commonly, the second input of the balanced set of inputs is grounded at the first and second frequencies of the mixer but not at DC. The second input of the balanced set of inputs is connected to a circuit having a first and second capacitor in parallel. In this circuit, one of the first and second capacitors is series resonant at the first frequency and the other capacitor is series resonant at the second frequency.
An example of a balanced mixer is a balanced Radio Frequency (RF) mixer. The RF mixer may frequency shift a Radio Frequency signal from a RF to an Intermediate Frequency where the RF signal is received from an antenna. As described above, one of the balanced set of input signals for the RF mixer is grounded at IF and RF. As also described above, the circuit used to ground this input includes a first capacitor in parallel with a second capacitor. The first capacitor is series resonant at IF and the second capacitor is series resonant at RF. In some implementations, the packaging of the first capacitor has a parasitic inductance at RF. In such an implementation, the first capacitor is effectively an inductor at RF. The first capacitor thus forms a parallel tuned circuit with the second capacitor at RF that increases the impedance to ground of the overall circuit.
This reduces the balance of the set of input signals and thus the gain of the output of the mixer. Thus, a need exists for a circuit that effectively grounds a signal at a first and a second frequency without inducing significant inductance at the higher of the first and second frequencies.